1. Field of the Invention
This invention provides a means for alterting a user that a seismic sensor has reached or exceeded a safe design depth.
2. Discussion of the Prior Art
In the art of geophysical exploration at sea, a ship tows a seismic streamer cable along an assigned line of survey. The streamer cable may be one or two miles long and may contain many hundreds of hydrophones. A acoustic pulse is generated in the water at or near the surface. The pulse travels downwardly into the sub-ocean strata whence it is reflected back towards the water surface where the reflected signals are detected by the hydrophones. Ordinarily, the streamer cable and its hydrophones is towed at a depth of 25 to 50 meters.
Typically, a hydrophone consists of two piezoelectric ceramic wafers cemented to thin metal diaphragms which in turn are sealed to the opposite ends of a short cylindrical spacer. The opposite surfaces of the wafers are usually silvered and the wafers are polarized. Electrical signal leads are soldered to the silvered surfaces. Alternatively, one side or pole of each wafer may be cemented to the metal diaphragm with conductive epoxy. The diaphragm becomes one terminal. The other terminal is formed by a single lead soldered to the other face, that is, the free pole, of each wafer. Water pressure variations applied to the hydrophones cause the wafers to flex, giving rise to electrical output signals in response to the varying applied pressures.
Hydrophones, such as above described, have definite design operating-depth limits. If subjected to an excess hydrostatic pressure, the wafers bend too far inwardly, they crack and are destroyed. One such hydrophone is taught by U.S. Pat. No. 3,660,809 issued 05/02/72 to R. Pearson.
The cracking problem due to excess over-pressure can be solved to some extent by inserting a plastic stop inside the cylindrical spacer. In the presence of an excessive pressure, the diaphragm and wafer are deformed inwardly but bottom out against the stop. See for example, U.S. Pat. No. 3,970,878 issued 08/20/76 to C. Berglund, which is incorporated herein by reference. Although the piezoelectric wafer may not actually crack when subjected to an unsafe pressure, the electrical output signals are distorted and the hydrophone loses sensitivity. Furthermore, the case of a '878 type hydrophone tends to acquire a permanent set or crumpling when subjected to an excess pressure. That effect further distorts the output signal.
Streamer cables are provided with depth sensors to monitor the average cable depth. The depth sensors may be mounted on the streamer cable at intervals of perhaps 1000 feet or more. In between the depth sensors, the streamer cable may sink or sag to an unsafe depth, due for example to an abrupt local reduction in water density or to a change in towing speed, yet the operator would not necessarily know that fact. In the case of a '809-type sensor, destruction of the hydrophone would result in a dead signal channel but by the time the operator discovers that situation, it is too late; the hydrophone is ruined. In the case of a '878 hydrophone, the channel would still be alive but the signal distortion likely would remain unrecognized.
It is a purpose of this invention to provide a hydrophone having self-contained means for warning an operator that the hydrophone has reached or exceeded a safe design depth so that the operator can take remedial action before signal degradation occurs and/or the hydrophone is destroyed.